Supplemental Security income benefits are only paid back to the month after you are found disabled. You will never be paid for the time before you apply, even if your inability to work because of medical problems started before you apply.
Disability Insurance benefits, on the other hand, can be paid as far back as the 12th month before you apply. Because Social Security does not pay benefits for the first five months after your disability is found to begin, you would have to be found disabled at least 17 months before your application in order to get the full year of back pay before you apply. Most people wait to apply for disability until after their disability begins and they stop working. For most people, there is a period of reckoning between when their work stops and the time they apply. If you know you are disabled and that it is going to be long term, it is good to apply as soon as possible.